NFL great Brandon Marshall, who last played football in 2018, is now targeting a boxing match with former WBC heavyweight champion Deontay Wilder.

TMZ Sports spoke to the 36-year-old - standing at 6'5 and 230-pounds. The now turned boxing enthusiast discussed his desire to step in the ring ... and his friend Evander Holyfield comparing Marshall to the 42-1-1 (41 KO's) Bronze Bomber.

"But what I'd say, I would challenge Deontay Wilder to dive a little deeper into the sweet science, and that's where I think I separate myself from a lot of these heavyweights."

Marshall is serious ... and says just because he doesn't have any pro fights under his belt, you shouldn't overlook his skills in the squared circle.

"You haven't seen me on pay-per-view. No, I don't have a win under my belt, or a loss, or a knockout. But at the end of the day, I know how to fight, I know how to box, I know the sweet science," Marshall said.

Marshall -- who says he likes Wilder and has learned from watching him box -- needs to fight this fall ... or he won't fight at all.

"If it doesn't happen before September, I'm gonna retire. I'm retiring from boxing. I'm retiring before I even step foot in the ring," Marshall said.

"If it doesn't happen, I'm just gonna move on to other things because I'm tired of chasing these guys."

No athletic commission in the United States would approve Wilder vs. Marshall as a sanctioned fight. And it's even difficult to imagine that contest being approved as an exhibition.

Wilder has not fought since losing to Tyson Fury in February 2020 ... and has a long list of potential opponents he could face in his return.

Marshall is the latest pro-athlete who wants to test the waters in boxing.

Last November, NBA veteran Nate Robinson made his pro boxing debut - and he was brutally knocked out in two rounds by Youtube star Jake Paul on the Mike Tyson-Roy Jones undercard.